Diene acyclic dendralenes

Dendralenes [24] (acyclic cross-conjugated polyenes) have been used as dienes in tandem Diels-Alder reactions, and a methodology for the synthesis of highly functionalized angularly anel-lated aromatic compounds has been developed (Scheme 16.23) [25]. A tandem double Diels-Alder reaction of DMAD with [3]dendralene followed by oxidation with DDQ gave the tetramethyl ester... 

We begin by considering the most conceptually simple case intermolecular DTDA reactions of acyclic [3]dendralenes. To date, the majority of studies have been conducted on 3-substituted systems as a consequence of their accessibility [4], and the control they offer over DA site selectivity. In the generalized example illustrated in Scheme 12.3, the [3]dendralene conformation 26 is disfavored due to the steric interaction between the terminal Z-hydrogen atom shown, and the inside substituent R. Thus, assuming the activation barrier for each DA reaction is approximately equal, the reaction should preferably occur at the s-cis diene site depicted in 25 as a result of its increased population. 

Only one study detailing DTDA reactions of the pseudo-acyclic 1,1- or 3,3-cyclo[3]dendralenes has been reported, most likely owing to the difficulties associated with DA reactions of such highly substituted dienes. The prototypical, and not yet reahzed, DTDA cascade of l,l-cyclo[3]dendralene 17 is presented in Scheme 12.12. If the first DA reaction occurs at the more sterically demanding exocyclic diene site, spiro-bicycle 65 is produced, which could react further to generate the... 

Cyclo [3] dendralenes contain both a semicyclic and acyclic diene site, and, in principle, either of the two tricyclic frameworks 92 or 94 can be accessed through alternative DTDA sequences (Scheme 12.19). 

The first (and only) detailed DTDA study of parent [4] dendralene (14) was reported by the Sherburn group in 2005 [35]. Upon addition of an excess of NMM, at ambient pressure and temperature, a mixture of five products was obtained in excellent overall yield (Scheme 12.26). The first DA reaction occurred primarily at the terminal diene site to provide l,2-monocyclic[3]dendralene 119, as well as 21% of the internal adduct 120. Triene 119 could not be isolated, and a second cycloaddition, predominately at the internal site, afforded the two endo-DA adducts 121 and 122. The major diastereomer arises via approach of the dienophile anti with respect to the succinimide ring in compound 119. The tris-adducts 123 and 124 originate from reaction at the acyclic diene site of 119, and from the less hindered top face (as drawn). The two intermediates generated... 

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